Internal combustion engines are well known. In operational cycle of the engine, fuel and air are drawn into a chamber by movement of a piston away from an inlet of the chamber, the fuel is then compressed by movement of the piston in an opposite direction—toward the inlet, a spark ignites the fuel forcing the piston away from the inlet and then the chamber is partially evacuated by the final piston movement toward an exhaust thereof near the inlet end. Though according to a simplified ideal theory a single spark will consume all of the fuel within the chamber in a near instantaneous fashion, this is not the case in reality.
Two prior art ignition systems that are very widely used are inductive discharge systems and capacitive discharge systems. The difference between the two systems relates mostly to an energy storage component used within each circuit where the inductive discharge system relies on an inductor and the capacitive discharge system relies on a capacitor. When using an inductive discharge based system energy tends to fall off at high revolutions per minute (related to strokes/minute) because an insufficient dwell time, to charge the coil, is provided. Further, a resulting low secondary voltage rise makes sensitivity to spark gap fouling significant. Typically, energy delivered to the spark plug gap is in range of 20–50 mJ at 1–2 ms of spark and has decaying power across its duration.
Capacitive discharge systems are known to release more spark energy over a relatively short period of time. Capacitive discharge systems produce up to 100 mJ of spark energy, but are characterized by limited spark duration of 150–500 μs. This very short spark duration results in significant difficulty igniting fuel during cold start conditions, with lean mixtures, and during transient behaviour of carburetion. Unfortunately, each of these systems provides only a single short duration spark, and as such, may fail to ignite all or a portion of the fuel within the chamber.
Multi-spark ignition systems represent an alternative to traditional inductive discharge and capacitive discharge systems. In a multi-spark ignition system, sparking occurs repetitively over a period of time. This has been shown to better influence combustion initiation—more reliably ignite the fuel within the chamber. When used on cold engines, multi-spark ignition systems typically more reliably start the engine. In multi-spark ignition systems, an energy discharge and charge cycle is created to charge and discharge a spark generation circuit to produce sparks at intervals and having similar profiles. Another approach to multi-spark is to discharge the spark generation circuit in a fashion resulting in an oscillation that oscillates below and above a sparking threshold resulting in periodic sparking during discharge.
Many multi-spark ignition systems rely on inductive discharge and as such provide lower energy discharge for a longer duration as disclosed, for example, in U.S. Pat. No. 6,397,827, wherein a high voltage is intermittently applied from an ignition coil for more than one time in a short time to generate sparks.
Multi-spark systems including those disclosed in U.S. Pat. No. 6,694,959 and U.S. Pat. No. 6,085,733 and high-frequency ignition systems as disclosed in U.S. Pat. No. 6,729,317 provide for increased overall sparking time during a stroke. The multi-spark ignition systems are able to maintain spark discharge above a desired energy level for a longer proportion of the stroke, in an interrupted and unipolar fashion. The high-frequency ignition systems are complex and produce a sinusoidal output voltage that reduces the formation of efficient plasma in the spark gap.
It would be advantageous to provide a spark discharge ignition system that overcomes the drawbacks of the prior art.